A hodgepodge summary of the self-excited oscillation of op-amp

These days in detail to see the next release of the self-excited vibration of the information, online also reviewed the relevant, made a summary as follows:

1. Oscillation of op-amp two conditions

1, loop gain greater than 1 (|af|>1)

2, feedback before and after the signal phase difference of more than 360 degrees, additional phase 180 or more (due to negative feedback connected to the opposite end).

A (open-loop gain) = Xo/xi

F (feedback factor) =xf/xo

2. How to judge the op-amp Oscillation:

The common use is the phase margin , i.e. when the 20lg|af|=0, whether the phase offset is more than 180

What is crossing frequency? Fc
The frequency at which the G (s) *h (s) corresponds to a gain of 1 (i.e. the amplitude constant) is the crossing frequency. Convert to DB Unit: 20log1=0db.

3. Ways to eliminate self-excitation:

From the self-excited oscillation conditions, the elimination of shocks can be started from two aspects:

(i) reduced loop gain (but this method increases op amp gain error)

For op amp, it is to reduce the feedback coefficient f, in other words, the greater the F, the greater the likelihood of a self-excited oscillation. For resistive feedback networks, the maximum value of F is the typical circuit of 1,f=1, which is the voltage-following circuit. This is the reason for the voltage following op-amp volatility (which is why we often see the op-amp manual labeled unity gain stability, but the gain error of the voltage following is small).

This is also true for voltage feedback, the capacitive load drive capability increases proportionally with the closed-loop gain. Therefore, if the closed-loop gain is 1 o'clock, the VFA can drive a 100pF capacitive load stably, then the closed-loop gain is 10 o'clock to drive 1000pF capacitive load

However, due to the design reasons, the size of the closed-loop gain is usually not easily changed, so the applicability of this method is not strong.

(ii) Increased phase margin

The phase shift of the open loop transfer function g (s) *h (s) should be maintained with 180° enough headroom at the crossing frequency point. How much is this margin selection appropriate? At present, 45 ° is usually selected in engineering application, i.e. the phase shift of the transfer function g (s) *h (s) should be less than 135 °.

Phase compensation:

Compensation based on compensating principle, delay compensation, advance compensation and hysteresis compensation

Hysteresis Compensation: Any compensation that causes phase shift hysteresis is called hysteresis compensation. Hysteresis compensation reduces the frequency of the main pole, that is, the amplifier band Narrows. (Similar to RC low-pass filter)----------curve ②

Advance compensation: Any compensation that makes phase shift ahead is called lead compensation, leading compensation causes the amplitude-frequency characteristic curve to appear 0 points, that is, the amplifier band becomes wider (like RC high-pass filter)-----curve ③

For the capacitive (CL) load present, the loop gain is reduced by the output resistor and CL. At the same time, there is no longer a proportional relationship between phase and gain, and phase lag becomes the decisive factor.

1. Off-loop compensation--advance compensation (for Yung load <1500pf or load impedance certain)

A resistor rx is strung between the output of the OP amp and the load capacitance, which generally has a resistance of 10-100 ohms.

2. In-loop compensation-advance compensation (for large capacitive load >1500PF or variable load impedance)

Rx is within the op amp feedback loop, while a feedback capacitor is connected in parallel to the feedback resistor (which eliminates the op amp input capacitance and the stray capacitance to form the poles), generally: rx=50~200ω,cf approx. 3~10pf

A hodgepodge summary of the self-excited oscillation of op-amp